This invention relates to a multi-board electronic system having a bus network and, more particularly, to an arrangement for switchably terminating the bus lines so that bi-directional signal drivers (i.e., transceivers) can be utilized.
A multi-board electronic system, such as a computer, typically includes a mother board, or backplane, having a plurality of edge connectors in which daughter boards are mounted. Printed circuit traces forming a bus network are provided on the mother board for interconnecting the connectors to provide communication paths between the various daughter boards. A desirable objective in such a system is to be able to space the daughter boards closely together in order to maximize the number of such boards supported on the mother board.
At one time, the spacing between daughter boards plugging into a mother board was determined by component height on the daughter board. The length of the printed circuit traces on the daughter boards from the bus drivers to the edge connector was determined by the most convenient parts placement on the daughter board. However, as clock and edge rates increase, the heavy capacitive loading of the buses and the ringing caused by long daughter board traces can cause severe degradation of signal quality and prevent the system from working properly.
It has been found that a low value series resistor inserted in the tap-off line, or stub, between a bus line and a receiver on the daughter board is effective in significantly reducing the signal degradation. However, the use of such a "damping resistor" is limited in that it must be at the receive end of the line. Placing a series resistor at the output of a driver to a bus line results in a voltage drop across the resistor, thereby sacrificing low level noise tolerance along the bus. The implication of this placement limitation is that the damping resistor approach is only useful for unidirectional lines. Many bus lines are bi-directional.
It is therefore a primary object of the present invention to provide a bi-directional implementation of the above-described simple resistive termination so as to allow its use in high speed digital busing applications.